WO2020039847A1

WO2020039847A1 - Image projection apparatus, diffusion screen, and mobile object

Info

Publication number: WO2020039847A1
Application number: PCT/JP2019/029457
Authority: WO
Inventors: Kazuhiro Fujita; Kazutaka Aboshi
Original assignee: Ricoh Company, Ltd.
Priority date: 2018-08-21
Filing date: 2019-07-26
Publication date: 2020-02-27
Also published as: JP2020030275A

Abstract

A diffusion screen (30) includes a micro-optical structure (300) being a two-dimensional array, disposed on one side of the diffusion screen (300) to be two-dimensionally scanned with a light beam. The micro-optical structure (300) is configured to diverge the scanning light beam at a divergence angle of 2θs degrees as a diverging light beam, and reflect, as a twice-reflected light ray, a light ray other than the scanning light beam to a side to which the diverging light beam is emitted. A conditional formula below is satisfied: θm > θs + θw + 90 degrees where θm denotes an angle formed by tangent planes defined at two positions at which the light ray is reflected twice by the micro-optical structure (300), θs denotes a half value of the divergence angle, and θw is greater than or equal to 0, and denotes an effective maximum scanning angle of the scanning light beam.

Description

IMAGE PROJECTION APPARATUS, DIFFUSION SCREEN, AND MOBILE OBJECT

Embodiments of the present disclosure relate to an image projection apparatus, a diffusion screen, and a mobile object.

An image projection apparatus is known that two-dimensionally scans a diffusing screen with a light beam having two-dimensional image information and forms a two-dimensional image on the diffusion screen. The image projection apparatus also causes the light beam to diverge so as to produce a diverging light beam by using the diffusion screen, and forms an enlarged virtual image by using an optical system for imaging an enlarged-virtual image, which is to be observed by the driver (see JP-6237124-B (JP-2014-139657-A)).
Such an image projection apparatus is commercialized as, for example, a head-up display (HUD), and it is becoming more popular for the image projection apparatus to be mounted on various kinds of drivable mobile objects, such as cars, trains, ships, helicopters, airplanes. Two-dimensional images are written in the diffusion screen by two-dimensional scanning of the diffusion screen with a light beam. That is, the light beam scans the diffusion screen two-dimensionally while being intensity-modulated in time series according to two-dimensional image information constituting a two-dimensional image.

The diffusion screen is capable of transmitting or reflecting the light beam, and is also capable of diverging the transmitted or reflected light beam to produce a diverging light beam. This action is referred to as a diffusion action of the diffuse screen. In order to achieve such a diffusion action, the diffusion screen has a refractive or reflective micro-optical structure two-dimensionally arranged at one side (any one of an incident plane, an exit plane of a light beam, and a reflecting plane). The refraction or reflection of the light beam by the micro-optical structure produces a diverging light beam.
In order to enable an effective observation of a two-dimensional image formed as an enlarged virtual image, the observer’s eyes have to be positioned within the imaging light flux for imaging the enlarged virtual image.
The area that enables the observer to observe an enlarged virtual image is referred to as an effective viewing area. If the observer's eyes are outside the effective viewing area, the observer is unable to visually identify an enlarged virtual image. In order to make it easy for the observer to observe an enlarged virtual image, the effective viewing area is preferably large to some extent to enable the observation even with the eyes of the observer slightly shifted. Further, the diffusion screen is used to enlarge the effective viewing area.

A larger effective viewing area can be obtained as the angle of divergence of the diverging light beam produced by the diffusion screen increases. However, with an increase in the effective viewing area, the size of the optical system for imaging an enlarged virtual image is likely to increase. In view of this, the angle of divergence is appropriately set according to the size of the image projection apparatus.

As described above, in the image projection apparatus, a two-dimensional image is formed on the diffusion screen, and the two-dimensional image is enlarged and formed as an enlarged virtual image by the optical system for imaging an enlarged virtual image. If external light (light from the outside of the image projection apparatus) enters the optical system from the enlarged-virtual-image side, the optical system serves to converge the external light, thus bringing the external light to a focus in a direction to the diffusion screen. Some of the external light might be reflected by the diffusion screen, and the reflected light is directed to the same direction as the direction of the diverging light beam. Then, the reflected light is diverged by the optical system, which might adversely affect the enlarged virtual image as noise light.

[PTL 1] JP- 6237124-B

Summary

Such noise light might reduce the contrast of the enlarged virtual image of the two-dimensional image, which is to be observed, thus hampering the observation of the enlarged virtual image.
To avoid such a situation, the diffusion screen may be tilted to shift the direction of the external light reflected by the diffusion screen, away from the optical path of the diverging light from the optical system. However, even if the diffusion screen is tilted, the reflection of light is likely to recur on the surface of the diffusion screen on which the micro-optical structure is arranged, thus failing to prevent the occurrence of noise light.

In view of the above, there is provided a diffusion screen including a micro-optical structure being a two-dimensional array. The micro-optical structure is disposed on one side to be two-dimensionally scanned with a light beam. The micro-optical structure is configured to diverge the scanning light beam at a divergence angle of 2θs degrees so as to emit a diverging light beam, and reflect, as a twice-reflected light ray, a light ray other than the scanning light beam to a side to which the diverging light beam is emitted. A conditional formula below is satisfied:
θm > θs + θw + 90 degrees
where
θm denotes an angle formed by tangent planes defined at two positions at which the light ray is reflected twice by the micro-optical structure,
θs denotes a half value of the divergence angle, and
θw is greater than or equal to 0, and denotes an effective maximum scanning angle of the scanning light beam.

Further, there is also provided a diffusion screen including a micro-optical structure that is a two-dimensional array, disposed on one side of the diffusion screen. The micro-optical structure is configured to transmit image light for forming an image two-dimensionally emitted to the micro-optical structure, and diverge the image light at a divergence angle of 2θs degrees so as to emit a diverging light beam. The micro-optical structure is further configured to reflect a light ray other than the image light to a side to which the diverging light beam is emitted. A conditional formula below is satisfied:
θm > θs + θw + 90 degrees
where
θm denotes an angle formed by tangent planes defined at two positions at which the light ray is reflected twice by the micro-optical structure, and
θs denotes a half value of the divergence angle.

The embodiments of the present disclosure provide a diffusion screen capable of effectively eliminating or reducing the adverse effect of external light reflected by surfaces arranged on the micro-optical structure.

The aforementioned and other aspects, features, and advantages of the present disclosure will be better understood by reference to the following detailed description when considered in connection with the accompanying drawings. The accompanying drawings are intended to depict embodiments of the present disclosure and should not be interpreted to limit the scope thereof. The accompanying drawings are not to be considered as drawn to scale unless explicitly noted.
FIG. 1 is an illustration for describing how a HUD is used as an example of an image projection apparatus. FIG. 2 is an illustration of an example configuration of the HUD in FIG. 1. FIG. 3 is an illustration of an example configuration of a light-source unit in FIG. 2. FIG. 4 is an illustration of an example of a micro-optical structure of the diffusion screen in FIG. 2, in which hexagonal microlenses are two-dimensionally densely arranged. FIGs. 5A and 5B (FIG. 5) are illustrations for describing the relation between the curvature of and the divergence angle at the lens surface of a microlens of the diffusion screen. FIG. 6 is an illustration for describing double reflection of external light on the microlens that is an example of the micro-optical structure. FIG. 7 is an illustration for describing a condition for preventing the double reflection of light rays on the microlens from adversely affecting the imaging of an enlarged virtual image. FIG. 8 is an illustration of light rays of double reflection on the microlenses in FIG. 7. FIG. 9 is an illustration of another example of the micro-optical structure. FIG. 10 is an illustration of a case in which the micro-optical structure is reflective. FIG. 11 is an illustration of external light not adversely affecting the imaging of an enlarged virtual image. FIG. 12 is another illustration of external light not adversely affecting the imaging of an enlarged virtual image. FIG. 13 is another example of the diffusion screen. FIGs. 14A and 14B (FIG. 14) is illustrations for describing how the twice-reflected light ray from the micro-optical structure of the diffusion screen is handled.

A description is provided of embodiments of the present disclosure below.
FIG. 1 is an illustration for describing how a HUD 1 is used as an example of an image projection apparatus.
In this example, the HUD 1 is used for a car (mo) as a mobile object, and is configured to display driving information to an observer Ob as a driver. In FIG. 1, the mobile object (a car) has a windshield 50, and the observer Ob has the eyes A.
A diverging light beam emitted from the HUD 1 strikes the windshield 50, and is reflected by the windshield 50. The reflected light beam is then directed toward the eyes A of the observer Ob. The enlarged image Im observed by the observer Ob is formed at a spatial position beyond the windshield 50 as viewed from the eyes A of the observer Ob.
FIG. 2 is an illustration of an example configuration of the HUD 1.
The HUD 1 includes a light-source unit 11, a light deflector 15, a mirror 20, and a diffusion screen 30, which are housed in a housing 10A and constitute a two-dimensional image generation unit. The HUD 1 further includes mirrors ML1 and ML2, and a concave mirror 40. The two-dimensional image generation unit, and these mirrors ML1 and ML2, and the concave mirror 40 are housed in a housing 1A. The two-dimensional image generation unit is sometimes referred to as an imager.

The light-source unit 11 generates a parallel light beam whose intensity is modulated in time series according to two-dimensional image information that constitutes a two-dimensional image, and emits the light beam toward the light deflector 15.
The light deflector 15 is, for example, a micro-electromechanical systems (MEMS) manufactured as a micro oscillating mirror element by a semiconductor process or the like. The light deflector 15 is configured to cause micro reflecting planes to two-dimensionally oscillate, and two-dimensionally deflect light rays from the light-source unit 11.
In some embodiments, a mirror system, in which two mirrors that oscillate or rotate about one axis are arranged with their axes orthogonal to each other, constitute the light deflector.
The two-dimensionally deflected light beam is reflected by the mirror 20, and the reflected light beam two-dimensionally scans the diffusion screen 30, thus writing a two-dimensional image onto the diffusion screen 30.
The two-dimensional image written to the diffusion screen 30 is hereinafter also referred to as an intermediate image.

Note that the diffusion screen 30 displays only pixels irradiated with a light beam at a moment, and the intermediate image is formed as a cluster of pixels displayed by the diffusion screen 30 at each moment.

The light beam incident on the diffusion screen 30 is converted into a diverging light beam by the diffusion screen 30, and exits the diffusion screen 30. Then, the light beam is sequentially reflected by the mirrors ML1 and ML2, and is directed to the concave mirror 40. The light beam reflected by the concave mirror 40 is directed to the windshield 50, and is reflected by the windshield 50 so that the reflected light beam is directed toward the eyes A of the observer. Thus, the enlarged virtual image Im is visually identified by the observer Ob.
The diffusing screen 30 serves to convert the light beam striking the diffusion screen 30 into a diverging light beam, and emits the diverging light beam. This action is referred to as a diffusion action.

In addition, the optical system for imaging an enlarged virtual image (the image forming optical system) is disposed between the diffusion screen 30 and the enlarged virtual image Im in the example of FIG. 2. The optical system is an optical component that serves to form an enlarged virtual image of the two-dimensional image on the diffusion screen 30. In the example of FIG. 2, the optical component includes the mirrors ML1 and ML2, the concave mirror 40, and the windshield 50. The concave mirror 40 and the windshield 50 (the windshield of an automobile typically has a concave shape with a concave surface facing the driver's seat) contribute to enlarging of a virtual image (an increase in the magnification of the virtual image).
The mirrors ML1 and ML2 are flat mirrors, and do not contribute to a change in the magnification of the enlarged virtual image.

In some embodiments, either one or both of the mirrors ML1 and ML2 are omitted depending on the layout of the optical system. Alternatively, a concave or convex curved mirror may be used for, for example, the mirror ML1 to contribute to an increase in the magnification of the virtual image, i.e., the image forming optical system.
Further, the image forming optical system may be another type of optical system for refracting light, in which one or more lenses are arranged, as a constituent optical component, instead of the optical system including only mirrors.
In some other embodiments, the optical system may have a partially reflective mirror, such as a combiner, same as the windshield 50.
In the example of FIG. 2, the concave mirror 40 is designed and arranged to correct a factor of the optical distortion in which the line in the horizontal direction of the enlarged virtual image is convex upward or downward due to the influence of the windshield 50. In still some other embodiments, the optical system may be designed to form an enlarged virtual image only at a windshield having a concave surface.

Next, an example configuration of the light-source unit 11 is specifically described with reference to FIG. 3.
The light-source unit 11(a light source) in FIG. 3 includes a plurality of

light source elements

111R, 111B, and 111G each having one or more light emitting points.
The light source elements are semiconductor laser elements (hereinafter referred to as laser diodes (LDs)), and emit light beams of different wavelengths: λR, λG, and λB. For example, the wavelength λR is 640 nanometer (nm), the wavelength λG is 530 nm, and λB is 445 nm.
Laser (light) beams λR, λG, and λB emitted from the light-source elements (LDs) 111R, 111G, and 111B pass through the

respective coupling lenses

112R, 112G, and 112B to be coupled to a subsequent optical system. The coupled light beams are shaped by the

aperture members

113R, 113G, and 113B, respectively.
The

aperture members

113R, 113G, and 113B have any shape, such as a circle, an ellipse, a rectangle, or a square, according to the divergence angle of the laser beam.
The laser beams shaped by the corresponding

aperture members

113R, 113G, and 113B pass through a combining element 115, and are combined into one light beam that travels along one optical path. The combining element 115 is a plate or prismatic dichroic mirror, and reflects or transmits each of the laser beams therethrough according to the wavelength of each of the laser beams, and thus combines the laser beams into one light beam that travels along one optical path. The combined light beam is directed to the reflecting plane of the light deflector 15 through the lens 116. The lens 116 is a meniscus lens with a concave surface facing the light deflector 15.

Depending on the design of the

coupling lenses

112R, 112B, and 112G, the lens 116 may not be used as long as an optimal diameter of the light beam is obtained at the diffusion screen 30. By controlling the blinking of the light-source elements (LDs) 111R, 111B, and 111G according to the two-dimensional image to be displayed, a parallel light beam of a desired color is emitted to the light deflector 15 (FIG. 2).

Next, an example configuration of the diffusion screen 30 is described.
As described above, the diffusing screen 30 has a micro-optical structure two-dimensionally arranged at one surface, which converts an incident light beam into a diverging light beam with a divergence angle of 2θs (degrees).
In the example of FIG. 4, hexagonal micro lenses 300 are two-dimensionally densely arranged on the incident-side surface (surface on which a light beam is incident) of the diffusion screen 30 as the micro-optical structure of the diffusion screen 30.
Each of the microlenses 300 causes incident light rays to diverge at a divergence angle of 2θs (degrees), thus producing diverging light rays (the microlenses 300 as a whole produce a diverging light beam). The diverging light rays diverge conically. The divergence angle of 2θs corresponds to the cone angle when the light rays diverge conically, and its half value, i.e., θs, is referred to as a half divergence angle.

Each of the microlenses 300 is a convex lens having a width of approximately 200 micrometers (μm) in the x-direction. The width is variable within a range of from approximately 50 to 300 μm according to the specification. Since the external shape of each microlens 300 is hexagonal, the microlenses 300 may be densely arranged. Note that the external shape of the microlens 300 of the micro-optical structure is not limited to a hexagon, and may be a rectangle or a triangle.
In the embodiment illustrated in FIG. 4, the microlenses 300 are regularly arranged in the micro-optical structure. However, the present disclosure is not limited to this configuration. Alternatively, the microlenses may be irregularly arranged such that the centers of the lenses may be decentered with respect to each other to form an eccentric array. In such a configuration, each microlens has a different shape.

In addition, the height at the vertex of the convex surface of each microlens 300 in the direction of the optical axis is variable. By randomly decentering the direction of arrangement of the microlenses or shifting the direction of the optical axis of a microlens, it is possible to eliminate moire due to periodical array or speckle caused by the interference of laser light passing through the boundary between adjacent microlenses.
The light beam that has reached the diffusion screen 30 scans the center of the microlens 300 two-dimensionally in the x-direction and the y-direction, for example, by raster scan. During this scanning, the emission of light beam is turned on and off to hit a plurality of dots, and gradation display is performed by, for example, turning on and off the emission of light beam. Alternatively, the intensity of the light beam may be modulated for gradation display.

The width of the microlens 300 (the distance between the two opposing sides) is preferably about 50 μm to 300 μm. However, no limitation is intended therein. The image area within the plane of the diffusion screen 30 is usually rectangular. However, no limitation is intended therein. Alternatively, the image area may be polygonal. The surface of the diffusion screen 30 does not have to be flat, and may be curved.

Next, the diffusion action of the diffusion screen 30 is described with reference to FIGs. 5A and 5B.
FIGs. 5A and 5B are two examples of the microlens array formed on the diffusion screen 30A/30B, each lens having a convex surface. In the diffusion screen 30A in FIG. 5A, each microlens LS1 has a small curvature (a large radius of curvature). In the diffusion screen 30B in FIG. 5B, each microlens LS2 has a large curvature (larger than the curvature of the microlens LS1) (a small radius of curvature, i.e., a smaller radius of curvature than that of the microlens LS1). A convex lens surface with a small curvature has a small refractive power, and a convex lens surface with a large curvature has a large refractive power.
As illustrated in FIGs. 5A and 5B, a light beam LB of the same diameter enters the

diffusion screens

30A and 30B. The microlenses LS1 and LS2 of the

diffusion screens

30A and 30B, respectively have different curvatures but has the same lens diameter.

In FIG. 5A, light rays LB1 and LB2 of the light beam LB are incident on the peripheral portion of the microlens LS1 having the small curvature. The incident light rays LB1 and LB2 are focused by the positive power of the microlens LS1, and diverges at a divergence angle of 2θs1 (a half-value divergence angle of θs1) as a diverging light beam DLB1.
In FIG. 5B, light rays LB1 and LB2 of the light beam LB are incident on the peripheral portion of the microlens LS2 having the large curvature. The incident light rays LB1 and LB2 are focused by the positive power of the microlens LS2, and diverges at a divergence angle of 2θs2 (a half-value divergence angle of θs2) as a diverging light beam DLB2.
As illustrated in FIGs. 5A and 5B, the divergence angle of 2θs1 of the microlens LS1 having the small curvature is smaller than the divergence angle of 2θs2 of the microlens LS2 having the large curvature. That is, the divergence angle increases as the curvature of the microlens increases.
In FIGs. 5A and 5B, the size of the microlenes LS1 and LS2 is the same. However, even if the microlens has the same curvature, when the lens pitch of the microlens array is large, an incident angle of the incident light rays with respect to the periphery of the microlens increases, which increases the refractive power, thus increasing the divergence angle. Accordingly, the divergence angle of 2θs is adjustable with the curvature (radius of curvature) and array pitch of the microlens.

The divergence angle 2θs that determines the diffusion action of the diffusion screen 30 in the HUD 1 is set according to an optical arrangement, such as an effective reflection area of the concave mirror 40, and a distance between the concave mirror 40 and the diffusion screen 30. The effective reflection area is an area in which the concave mirror 40 receives a virtual-image forming light beam diverged by the diffusion screen 30. In other words, the divergence angle 2θs is set according to the boundary of the effective viewing area for the enlarged virtual image.
That is, in the microlens array of the diffusion screen 30, the array pitch is determined according to a desired pixel density of an enlarged virtual image, and the divergence angle of 2θs is determined according to, for example, the effective reflection area of the concave mirror 40 and the distance between the concave mirror 40 and the diffusion screen 30. The curvature (radius of curvature) and the array pitch of the microlens array are adjusted to achieve such conditions.

The above description is given with reference to the HUD 1 in FIG. 2.
The following describes the external light described above.
As described above, external light is defined as light from the outside of the image projection apparatus that strikes the image forming optical system, from the enlarged virtual image side. Examples of the external light may include various types of light beams. In the example of the HUD 1 used in a vehicle (an automobile) as illustrated in FIG. 2, the external light is sunlight during the daytime, and is illumination light in the nighttime. Among these examples, the sunlight has the strongest intensity and might cause a trouble.
The HUD 1 is often disposed near the windshield 50. As illustrated in FIG. 2, when the sunlight is within the optical path of light for forming an enlarged virtual image, the direct sunlight NzL comes into the HUD 1. When the HUD 1 is mounted on a vehicle, the direct sunlight NzL is likely to come into the HUD 1 with a certain probability.

In FIG. 2, the direct sunlight NzL of a luminous flux Dnz1 that has struck the concave mirror 40 as a parallel light rays is reflected by the concave mirror 40, and the reflected light rays are converged by the concaved mirror 40, thus producing a light beam of a luminous flux Dnz2. The converged light enters the diffusion screen 30. Such sunlight NzL that has entered the diffusion screen 30 has an extremely strong intensity unless attenuation is made before entering the diffusion screen 30, because the sunlight Nzl is converged by the concave mirror 40.
The visible light reflectances of the concave mirror 40, and the mirrors ML1 and ML2 are set to extremely high values because light projected for forming an enlarged virtual image is preferably as bright as possible.

For this reason, some light rays of the sunlight that has struck the diffusion screen 30 are reflected by the diffusion screen 30, and the reflected light rays are likely to be directed to the effective viewing area of the observer Ob through the optical path of light for forming an enlarged virtual image. Thus, noise light adverse to the enlarged virtual image occurs.
Such noise light adversely reduces the contrast of the enlarged virtual image, and hampers the visual identification of the enlarged virtual image.

When a HUD is used in an automobile as in the example of FIG. 1, the following phenomenon known as postcard effect due to the noise light occurs.
A great advantage of mounting a HUD on an automobile is that the vehicle's information, navigation information, warning, etc. are displayed through the windshield to minimize the driver's gaze movement even while driving, which achieves a comfortable and safe driving environment.
There are cases in which one or more marks indicating the position of a preceding vehicle are also displayed as an enlarged virtual image by the HUD based on information acquired by the front camera mounted on the driver’s vehicle.
In such a case, by displaying the enlarged virtual image a few tens of meters ahead of the driver’s vehicle, which is substantially equal to the distance to the preceding vehicle, a natural-looking display where the driver does not have to adjust the focus of the eyes is implemented. However, with an increased in the distance to the position at which the enlarged virtual image is displayed, the size of the image forming optical system increases, which make it difficult to mount on a vehicle. Thus, the enlarged virtual image cannot be displayed several tens of meters ahead of the driver’s vehicle.

For this reason, a typical image forming optical system is designed to be optimized for natural looking display such that the driver does not feel uncomfortable with the distance to external information, and the enlarged virtual image is displayed at an appropriate distance from the driver.
To enable the driver to not feel uncomfortable with the distance to a target object at a long distance (for example, a preceding vehicle), an enlarged virtual image is displayed (formed) 3 meters (m) ahead, more preferably 4 m or more ahead of the observer Ob (driver).
In a HUD whose imaging position is designed to be 4 m ahead of the eyes of the observer, for example, even if the mark information (information regarding the position of the preceding vehicle) and direction-change (turn) information are actually displayed as an enlarged virtual image 4 m ahead of the observer, the mark information and turn information appear to be several tens of meters ahead, at the same position as that of the preceding vehicle so that the observer can observes the mark information and turn information without uncomfortable feelings.
This is a type of optical illusion related to the cognitive function of human, in which the displayed information is perceived to be attached to the preceding vehicle to form a single unit.
However, in fact, the enlarged virtual image is designed to be formed at a position of 4 m ahead of the observer Ob (the actual imaging position).
Such a type of optical illusion occurs when the contrast of the enlarge virtual image (the mark information and turn information) is sufficiently high. When the contrast of the enlarged virtual image decreases to some extent, the enlarged virtual image appears at the actual position (4 m ahead of the observer Ob).
The size of the enlarged virtual image is about the size of a typical postcard.
When the low-contrast enlarged virtual image appears at the actual imaging position, the image perceived to be attached to the preceding vehicle to form a single unit appears to be of the size of a postcard and separate from the preceding vehicle. That is, the postcard displaying information floats up from the preceding vehicle and is observed by the observer Ob. Such a phenomenon is referred to as postcard effect.
When the postcard effect occurs, the observer Ob feels uncomfortable due to the separation of the display information from the preceding vehicle.

The contrast of the enlarged virtual image decreases due to noise light generated by the external light reflected by the diffusion screen. In view of this principle, the postcard effect can be effectively eliminated by preventing the reflected light as noise light from contributing to the imaging of an enlarged virtual image.

The diffusion screen 30 according to an embodiment of the present disclosure has the micro-optical structure capable of diffusing light incident on the diffusion screen 30. With this configuration, the external light that has been reflected twice can be prevented from contributing to the imaging of an enlarged virtual image, i.e., becoming noise light adverse to an enlarged virtual image to be formed.

With reference to FIG. 6, the reflection of external light (exemplified by sunlight) by the micro-optical structure is described. Further, a two-dimensional microlens array is assumed as the micro-optical structure.
The upper side of FIG. 6 is the light-source side, that is, the light-deflector side in the example being described, and the lower side is the image-forming optical system side, that is, the concave-mirror side.
A diffusion screen 30C has a smooth surface at one side (the lower-side surface in FIG. 6), and a two-dimensional grid pattern array at the other side (the upper-side surface in FIG. 6). In the two-dimensional grid pattern array, microlenses LS each having a constant radius of curvature are arranged at a predetermined array pitch without any gaps between the adjacent microlenses. The light beam two-dimensionally deflected by the light deflector travels toward the microlens array in a direction from the upper side to the down side of the drawing.

A light ray LB1 in FIG. 6 refers to one of the rays constituting the light beam incident on the microlens array at this time. The light beam including the light ray LB1 is incident perpendicularly to the diffusion screen 30C, and the light ray LB1 itself is refracted and travels to the right of the drawing due to the diffusion action of the diffusion screen 30C. The angle θs is the half-value divergence angle of the diverging light ray produced by the diffusion screen 30C.
Although the light beam is actually scanned in a two-dimensional manner, it is assumed that the light beam is deflected and scanned in the right-to-left direction within the plane parallel to the drawing sheet for a specific description. In FIG. 6, the angle θ denotes an angle at which the light beam is incident on the diffusion screen 30C (an angle relative to the vertical direction of the drawing), which is referred to as a scanning angle θ. The scanning angle θ is variable with the scanning of the light beam, and the maximum scanning angle satisfies the following relation within the effective image area of the two-dimensional image written in the diffusion screen 30C:
-θmx ≦ θ ≦ +θmx (θ is greater than or equal to -θmx, and less than or equal to +θmx).
In the example of FIG. 6, θ is 0. The absolute values of -θmx and +θmx may not be equal to each other, but for the sake of simplicity of description, the absolute values of -θmx and +θmx are assumed to be equal to each other. The absolute value of θmx is referred to as an effective maximum scanning angle θw.

The sunlight, which is external light, comes to the diffusion screen 30C from the image-forming optical system side (from the lower side of FIG. 6) toward the information. The sunlight itself is a substantially parallel light beam. However, since the sunlight is converged by the concave mirror 40 before striking the diffusion screen 30C, one of the light rays of the sunlight may be a sunlight ray SL1 that strikes the diffusion screen 30C perpendicularly to the plane of the diffusion screen 30C, and another light ray may be a sunlight ray SL2 that obliquely strikes the plane of the diffusion screen 30C.
Note that FIG. 6 is a simple illustration for a brief description, and the behavior of the light beam is not truly depicted. In the continuous plane on the observer side, the sunlight ray SL2 obliquely incident on the plane is actually refracted according to the incident angle, like the external light beam. However, if the ray is incident on and exits the continuous plane, the refraction of ray is cancelled out. Accordingly, the refraction of ray on the plane is omitted in FIG. 6. The same applies to the microlens array described below.

In FIG. 6, the sunlight ray SL1 as external light strikes the lens surface of the microlens LS, and is reflected by the lens surface twice at reflection points RF1 and RF2. A tangent plane CP1 is a plane in contact with the microlens surface at the reflection point RF1, and a tangent plane CP2 is a plane in contact with the microlens surface at the reflection point RF2. The tangent plane CP1 and the tangent plane CP2 form an angle θm (hereinafter, referred to also as a crossing angle θm).
When the sunlight is reflected by the lens surface of the microlens LS twice, the crossing angle θm is also produced by the sunlight SL2. In other words, the crossing angle θm is also formed by the tangent planes of the reflection points of the sunlight SL2.

In the example of FIG. 6, the crossing angle θm is 90 degrees, and the sunlight rays SL1 and SL2 are reflected by the lens surface in the incident directions of the sunlight rays LS1 and SL2, thus becoming the light rays LP1 and LP2 reflected twice, respectively. That is, the twice-reflected light rays LP1 and LP2 are retroreflected light (rays). In this case, the lens surface of the microlens LS serves as a retroreflecting surface of the sunlight rays.
If the lens surface of the microlens serves as a retroreflecting surface, the twice-reflected light rays LP1 and LP2 travel back in an opposite direction of the incident direction through the incident optical path, through which the sunlight rays SL1 sand SL2 have traveled to reach the diffusion screen 30C. Then, the traveled-back light rays LP1 and LP2 strikes the concave mirror 40. The light rays LP1 and LP2 reflected by the concave mirror 40 overlap the enlarged virtual image formed by the image-forming optical system, which adversely reduces the contrast of the enlarged virtual image.

The reduction in the contrast of the enlarged virtual image might cause the postcard effect as described above.

In FIG. 6, each of the exit angles of the two-reflected light rays LP1 and LP2 (the angles relative to the up-to-down direction of the drawing) is smaller than the divergence angle θs of the light beam incident on the diffusion screen 30C from the light-source side. With this configuration, the twice-reflected light rays LP1 and LP2 do not go out of the optical path of the light incident on the diffusion screen 30C.
The inventors have found the conditions for preventing the light rays reflected by the microlenses (micro-optical structure) twice from adversely affecting the imaging of an enlarged virtual image through studies on the above-described phenomenon.

In FIG. 7, a sunlight ray Sl as external light enters the diffusion screen 30D from the lower side of the drawing at an angle (incident angle) of θi, and strikes the lens surface of a microlens LS0. Then, the sunlight ray SL is reflected by the lens surface twice at reflection points RF1 and RF2, and the reflected light ray LP exits the diffusion screen 30D at an angle (exit angle) of θr relative to the plane of the diffusion screen 30D. As illustrated in FIG. 7, a light ray LB1 constituting a light beam exits the diffusion screen 30D at a divergence angle of 2θs (θs FIG. 7 is a half-value divergence angle).
Same as in FIG. 6, a tangent plane CP1 is a plane in contact with the microlens surface at the reflection point RF1, and a tangent plane CP2 is a plane in contact with the microlens surface at the reflection point RF2. The tangent plane CP1 and the tangent plane CP2 form an angle θm (hereinafter, referred to also as a crossing angle θm).

FIG. 8 is an illustration for describing the relation of the crossing angle θm, the incident angle θi, and the exit angle θr described above.
In FIG. 8, the reference line is a straight line orthogonal to the lower-side plane in FIG. 7.

The following conditional formula is satisfied where θi is an incident angle of sunlight as external light, θr is an exit angle of the twice-reflected light ray (an angle formed by the twice-reflected light ray LP and the reference line), θir is an angle formed by the incident sunlight SL and the twice-reflected light ray LP, θm is a crossing angle formed by the tangent plane CP1 at the reflection point RF1 and the tangent plane CP2 at the reflection point RF2, θ1 is an angle of the sunlight SL incident on the tangent plane CP1, which is equal to the reflection angle of the sunlight SL at the reflection point RF1, θ2 is an angle of the sunlight SL incident on the tangent plane CP2, which is equal to the reflection angle of the sunlight SL at the reflection point RF2:
θm = 180° - (θ1 + θ2) θir = 180° - 2(θ1 + θ2) = θi + θr.

Accordingly, the exit angle θr of the twice-reflected light ray LP satisfies the formula below: θr = θir - θi = 180° - 2(θ1 + θ2) - θi = 180° - 2(180° - θm) - θi = 2θm - θi - 180°.
When the reflection angle θr of the twice-reflected light ray LP is greater than the half-value divergence angle θs in FIG. 7, the twice-reflected light ray LP ceases to reach the effective reflection area of the concave mirror 40, which successfully prevents the adverse effect of noise light on the imaging of the enlarged virtual image.
When the conditional formula: 2θm - θi - 180° > θs is satisfied, the relation: θr > θs is achieved. Accordingly, the conditional formula: θs2θm > θi + 180° + θs, i.e., θm > (θi + θs)/2 + 90° is a condition for satisfying the relation: θr > θs where θi denotes the incident angle of the sunlight SL, and θs denotes the half-value divergence angle.

As described above, the incident angle θi is an angle at which the sunlight SL as external light that has been converged by the concave mirror 40 is incident on the diffusion screen 30D. The incident angle θi is greater than or equal to 0 and less than or equal to θs (0 ≦ θi ≦ θs). This is because the light beam diverged by the diffusion screen at the half-value divergence angle θs is received by the concave mirror.

Accordingly, when the incident angle θi is 0 (the minimum value), the conditional formula (1) below is satisfied:
θm > θs/2 + 90° (1)
When the incident angle θi is the maximum value θs, the conditional formula (2) below is satisfied:
θm > (θs + θs)/2 + 90° = θs + 90°, that is, θm > θs + 90° (2).

Since the conditional formula: θs + 90° > θs/2 + 90° is satisfied, the range of θm where both θs and θm satisfy the conditional formulae (1) and (2) simultaneously is θm > θs + 90°.

In the above-described example, the scanning angle θ of a light beam is assumed to be 0. When the scanning angle θ is not 0, the conditional formula: θm > θs + θ + 90° (the condition for the half-value divergence angle θs) is satisfied so as to prevent the twice-reflected light ray LP from reaching the effective reflection area of the concave mirror. Accordingly, when the conditional formula (3) below (the condition for the effective maximum scanning angle θw) is satisfied, the twice-reflected light ray LP is prevented from reaching the effective reflection area of the concave mirror:
θm > θs + θw + 90° (3).

Thus, a microlens array is designed to have a lens surface whose curvature satisfies the following conditional formula where θm denotes a crossing angle formed by two tangent planes of two reflection points at which any light ray is reflected twice (a first reflection occurs at one reflection point, and a second reflection occurs at the other reflection point) by the lens surface of the microlens, θs denotes a , half-value divergence angle formed by the micro lens array, θw denotes a maximum effective scanning angle: θm> θs + θw + 90
With this configuration, the twice-reflected light ray is prevented from overlapping the light forming an enlarged virtual image when emitted from the HUD. Accordingly, the reduction in the contrast of the enlarged virtual image due to sunlight as external light is reduced or eliminated, thus preventing the occurrence of postcard effect.
For example, assuming that the divergence angle 2θs is 40°, and the effective maximum scanning angle θw is 15°, the half-value divergence angle θs becomes 20°. Accordingly, by setting θm to 125° (= 20 + 15 + 90) or more, the twice-reflected light ray LP is prevented from reaching the effective reflection area of the concave mirror.

In the above-described example, the microlens array has a convex surface on the light-deflector side. However, this is just one example. In another example, the microlens array has a concave surface on the image-forming optical system side (the image-forming optical system is hereinafter referred to also as an observation system). In such a case as well, external light might be reflected by the lens surface twice. Further, as described later, the embodiments of the present disclosure are applicable to a reflective diffusion screen as a micromirror array having concave surfaces.

Although the microlens (microlens array) is described above as the example of the micro-optical structure, this is just one example. As illustrated in FIG. 9, the operational effects of the embodiments of the present disclosure can be exhibited by using a prism array as well.
The planes constituting the prism correspond to the tangent planes CP1 and CP2. Accordingly, the crossing angle θm is the angle of the prism, and the angle of prism is set so as to satisfy the conditional formula (3): θm > θs + θw + 90° where θs denotes the half-value divergence angle, and θw denotes the maximum effective scanning angle.

Although the example in FIG. 9 is drawn two-dimensionally, the micro-optical structure may be three-dimensionally conical or may have a composite surface whose apex portion is curved.

Further, in the above-described example, the light beam is emitted to the diffusion screen, and the light beam transmitted through the diffusion screen is caused to diverge under the diffusion action of the diffusion screen, thus becoming a diverging light beam. Such a light-transmissive diffusion screen is just one example of the diffusion screen. Alternatively, a reflective diffusion screen may be used.

In the reflective diffusion screen for example, a light beam is two-dimensionally deflected by the light deflector, and the deflected light beam obliquely strikes the surface of the micro-optical structure of the diffusion screen.
Then, the diverging light beam reflected by the diffusion screen under its diffusion action is separated from the light beam incident on the diffusion screen.

The diverging light beam travels in a direction in which projected light travels through the image-forming system, along the optical path of the projection light. The external light travels in the opposite direction of that of the diverging light beam through the optical path. When the external light is simply reflected by the diffusion screen, most light rays of the external light travel toward the light-deflector side. Thus, few components of the external light return to the image-forming optical system side.

However, some light rays of the external light components are reflected twice by the micro-optical structure, and return to the image-forming optical system side, which adversely affects the imaging of an enlarged virtual image.
In order to eliminate the adverse effect of the components of the twice-reflected external light, the diffusion screen is configured to have the micro-optical structure in which concave surfaces are formed on the side to be two-dimensionally scanned with a light beam. The micro-optical structure causes the two-dimensionally scanning light beam to be reflected at the divergence angle 2θs, thus producing a diverging light beam (the scanning light beam is reflected by the micro-optical structure at the divergence angle 2θs, thus becoming a diverging light beam). In this case, the following conditional formula is satisfied by the exit angle θr, the half-value divergence angle θs, and the effective maximum scanning angle θw of the external light reflected twice by the micro-optical structure: θr > θs + θw

FIG. 10 is an example of the micro-optical structure as a two-dimensional array of micro-concave reflecting surfaces, disposed on the incident side. The detailed description is given with reference to FIG. 10.
In the diffusion screen, the two-dimensional array of the micro-concave reflecting surfaces RS0 is formed on one side of an appropriate base material BS. In FIG. 10, the lower side corresponds to the image-forming optical system side (observation-system side). The light beam (one of the light rays constituting the light beam two-dimensionally scanning the micro-concave reflecting surface RS0 array is a light ray LB1) strikes the micro-concave reflecting surface RS0 from the lower side, and is reflected by the micro-concave reflecting surface RS0, thus turning a diverging light beam.
The half-value divergence angle θs of the divergence angle 2θs of the diverging light beam is as illustrated in FIG. 10. In the example of FIG. 10, the scanning angle θ is 0.
Further, the sunlight SL strikes the micro-concave reflecting surface RS0 at the incident angle θi, and is reflected twice by the micro-concave reflecting surface RS0. The twice-reflected light ray LP exits the micro-concave reflecting surface RS0 at the exit angle θr.
As apparent from FIG. 10, when the scanning angle θ is 0, by satisfying the following conditional formula, the twice-reflected light ray travels toward the outside of the range in which the diverging light beam travels (the twice-reflected light ray travels in a direction shifted outward from the direction of travel of the diverging light beam), thus preventing the adverse effect on the imaging of an enlarged virtual image: θr > θs
Further, the micro-concave reflecting surface RS0 is configured such that the following conditional formula is satisfied by the effective maximum scanning angle θw, the crossing angle θm, and the half-value divergence angle θs:
θm > θs + θw + 90
The crossing angle θm is formed by the tangent planes defined by the reflection points RF1 and RF2 of the micro-concave reflecting surface RS0 at which the external light is subsequently reflected (the first reflection occurs at the reflection point RF1, and the second reflection occurs at the reflection point RF2), as described above.
Such a configuration can be obtained by adjusting at least one of the curvature and the pitch of the micro-concave reflecting surfaces RS0.

In a reflective diffusion screen, the reflectance is set to a great value, and the reflectance of the external light is substantially equal to the reflectance of the light beam emitted to the diffusion screen. Thus, the advantageous effect of the embodiment is exhibited significantly.
A detailed description is given with reference to FIGs. 11 and 12.
In FIGs. 11 and 12, the same numeral references and symbols are applied to the light deflector and the concave mirror as in FIG. 2. It is assumed that the diffusion screen 30D in FIGs. 11 and 12 has the microlens array according to the embodiment illustrated in FIG. 7.
The light beam LB deflected by the light deflector 15 is two-dimensionally deflected in a direction parallel to the drawing sheet and a direction orthogonal to the drawing sheet so as to scan the diffusion screen 30D two-dimensionally.
In this scanning, directions orthogonal to each other are referred to as a main scanning direction and a sub-scanning direction.

In FIGs. 11 and 12, the main scanning direction is parallel to the drawing sheet, and the sub-scanning direction is orthogonal to the drawing sheet. The two-dimensional scanning is performed by, for example, raster scanning.
In general, if the two-dimensional image to be displayed is a horizontally-oriented screen, the main scanning direction is set to the horizontal direction, and the scanning angle of the main scanning is increased.
The effective-full-scanning angle 2θw is illustrated in FIG. 11. If the scanning angle in the sub-scanning direction is greater than in the main scanning direction, the effective-full-scanning angle 2θw is applied to the scanning angle in the sub-scanning direction in the following description. The maximum effective scanning angle θw is also illustrated in FIG. 11.

The light beam LB deflected by the light deflector 15 scans the effective display area on the diffusion screen 30D between the A region and the B region in FIG. 11, so as to draw a two-dimensional image therein. The light beam LB is not a light ray but a bundle of light rays having a finite diameter (for example, 50 μm).
When the light beam LB is incident on the A region of the diffusion screen 30D at the scanning angle θw, the light beam LB is diverged by the diffusion screen 30D under its diffusion action, thus turning a diverging light beam DLBA having a half-value divergence angle θs (i.e., a divergence angle 2θs). That is, the deflection range of the diverging light beam corresponds to the angle of “θw + θs” on the A-region side.
In the image-forming optical system as illustrated in FIG. 2 for example, the diverging light beam that has been deflected for scanning travels toward the concave mirror 40. The concave mirror 40 receives the light rays that have scanned the scanning range of “θw + θs” in the main scanning direction on the A-region side.
Similarly, when the light beam LB is incident on the B region of the diffusion screen 30D at the scanning angle θw, the light beam LB is diverged by the diffusion screen 30D under its diffusion action, thus turning a diverging light beam DLBB having a half-value divergence angle θs (i.e., a divergence angle 2θs). That is, the deflection range of the diverging light beam corresponds to the angle of “θw + θs” on the B-region side as well. Accordingly, the concave mirror 40 receives the diverging light beam that has been deflected in the deflection area corresponding to the angle of “2(θw + θs)”. This angle is referred to as an effective marginal angle θLM as illustrated in FIG. 11.

Note that, in scanning in the main scanning direction, the incident angle of the light beam LB on the diffusion screen 30D differs between the A region and the B region. In addition, the divergence angle 2θs slightly differs between the A region and the B region due to aberration generated in the lens surface of the microlens. However, it is assumed that the divergence angle 2θs is the same between the A region and the B region, for the sake of simplicity.

Referring to FIG. 11, the external light (sunlight SL) strikes the concave mirror, and then strikes the A region of the diffusion mirror 30D. In this case, the sunlight SL has the effective marginal angle of “θw + θs” corresponding to the range of the diverging light beam to be received by the concave mirror when striking the A region of the diffusion mirror 30D.

The twice-reflected light beam LP that has been reflected twice by the lens surface of the microlens in the A region, proceeds while forming the angle θr as illustrated in FIG. 11. If the angle θr is greater than “θs + θw” obtained by adding the scanning angle θw to the half-value divergence angle θs of the diverging light beam DLBB in the B region, the twice-reflected light beam LP travels to the outside of the range of the effective marginal angle of the concave mirror, thus preventing the adverse effect on the imaging of an enlarged virtual image.

As described above, the following conditional formula is satisfied by the angle θr, the incident angle θi of the external light (sunlight SL), and the crossing angle θm: θr = 2θm - θi - 180° By substituting “θw + θs” into the incident angle θi, the conditional formula below is obtained to satisfy the relation of “θr > θs + θw”: θr = 2θm - (θw + θs) - 180°
Then, the conditional formula below is obtained: 2θm - θw - θs - 180° > θs + θw That is, the conditional formula below is obtained: 2θm - 180° > 2θs + 2θw.

Accordingly, the conditional formula below is obtained: θm > θs + θw + 90°

At this time, the light beam, i.e., the sunlight SL (external light) reflected by the concave mirror and incident on the diffusion screen 30D, strikes the A region at the maximum incident angle of “θw + θs”. If the twice-reflected light ray LP of the sunlight SL has an angle greater than or equal to the maximum angle of “θw + θs” of the diverging light beam DLBB on the B region side (opposite the A region side), the twice-reflected light beam LP travels to the observation side while forming a greater angle than that of the diverging light beam DLBB. Thus, such a light ray of the sunlight SL can be prevented from reaching the observer Ob.

It is preferable to determine any conditions for creating such a situation.
That is, it is satisfactory as long as each microlens of the effective display area constituting the diffusion screen satisfy the conditional formula: θm > θs + θw + 90°

If some areas of the microlens array cause the twice-reflected light ray to enter the observation system and some other areas cause the twice-reflecteed light ray to not enter the observation system, various conditions for incident external light are considered. However, by adopting such a configuration, irrespective of how the external light enters the observation system, any factor for generating the postcard effect can be eliminated from any part of the diffusion screen, thus providing the apparatus capable of displaying a high-quality image.

FIG. 12 is an illustration for describing the relation between the concave mirror 40 as a part of the observation system and the description with reference to FIG. 11.
A region 40OED outside the above effective marginal angle θLM, i.e., 2(θw + θs) of the concave mirror 40 is indicated at the A region. The twice-reflected light ray LP that has been reflected twice at the B region travels toward the region 40OED. Thus, such a reflected light ray does not adversely affect the imaging of an enlarged virtual image.

To supplement the above description, the light beam LB deflected by the light deflector 15 scans the diffusion screen 30D. Then, the light beam LB is caused to diverge by the diffusion screen 30D under its diffusion action, thus turning a diverging light beam. The diverging light beam as projection light travels to the concave mirror 40 having an effective reflection area that captures substantially all the projection light.
At the same time, the external light such as the sunlight SL enters the image-forming optical system, and travels in a direction opposite the direction of travel of the projection light while converging to the diffusion screen 30D. The range in which the concave mirror 40 captures the external light corresponds to the effective reflection area. In fact, although, in some cases, the sunlight might not directly strike on the effective reflection area by blocking the sunlight with the dust cover of HUD, the bezel member used in mounting component on the vehicle, the roof part of vehicles, etc., the external light, such as sunlight, usually enters the image projection apparatus, and is reflected by the concave mirror 40 within the effective reflection area. Then, the reflected light travels to the diffusion screen 30D in the opposite direction of the diverging light beam, thus striking the diffusion screen 30D. The external light that has struck the diffusion screen 30D is reflected twice by the lens surface of the microlens constituting the diffusion screen 30D, and travels back to the observation system. However, if the twice-reflected light ray travels to the outside of the effective reflection area of the concave mirror 40, such a light ray (external light) does not reach the observer.

Note that, even if the twice-reflected light ray strikes the effective reflection area, such a light ray might not reach the effective viewing area (eye range) of the observer in the following case. If the twice-reflected light ray is incident on the concave mirror 40 at a large incident angle, and is reflected by the concave mirror 40, the reflected light ray does not reach the effective viewing area of the observer. Thus, the observer does not identify such a light ray.
By applying the diffusion screen according to the embodiments of the present disclosure to the image projection apparatus, even if the external light reaches the diffusion screen, the twice-reflected light rays of the external light can be prevented from traveling to the observer. Thus, the image projection apparatus, such as a HUD, equipped with the diffusion screen according to the embodiment of the present disclosure is capable of generating an enlarge virtual image without a reduction in contrast and without the occurrence the postcard effect.

In addition, the external light that has reached the diffusion screen includes other light rays other than the light reflected by the concave mirror. According to the Fresnel reflection rule, with an increase in the incident angle of light incident on the diffusion screen, the reflectance increases, which increases the intensity of the reflected light. In the above-described example, if the sunlight as external light enters the HUD and is not reflected by the concave mirror, some light rays of the sunlight might be reflected by other mirrors constituting the image-forming optical system, and reach the diffusion screen.
Further, some light rays of the sunlight might not enter the image-forming optical system from the opposite direction of the direction in which the light beam forming an enlarge virtual image travels. Such light rays might obliquely strike the image projection apparatus, and illuminate other components inside the housing of the HUD, instead of striking the concave mirror to be reflected to the diffusion screen. Then, the illuminated component might serve as a secondary light source to emit the light to the diffusion screen.

The diffusion screen according to the embodiments of the present disclosure is extremely effective for such light rays.

In the above-described example, the diffusion screen has the microlenses as an example of a micro-convex-and-concave structure. This is just one example. Each microlens of the microlens array may not have the same radius of curvature, and may have an aspherical shape. Alternatively, microlenses of the lens array may have a lens curve such that the curvature is different between the vertical direction and the horizontal direction. Specifically, the height is constant, and the array pitch is different between the vertical direction and the horizontal direction in the microlenses. Further, although the lens size is different between the microlenses, the sag amount, i.e., the difference between the lens boundary portion and the lens vertex portion, is the same between the vertical array direction and the horizontal array direction of the microlenses.

Although the surface of the microlens array is subjected to processing such as antireflection to minimize the reflection of light, it is impossible to reduce the reflection of light to 0% from the viewpoint of cost and mass productivity of the diffusion screen. In addition, one of the reflection points of the twice-reflected light ray might meet the total reflection condition of Fresnel because the incident angle at the point is larger. Accordingly, even if the anti-reflection coating is applied to the lens surface, the light ray is reflected at a high reflectance or totally reflected at one of the two reflection points to return because the lens surface is optimized according to the incident angle of the projection light.

Further, the sunlight that has struck the diffusion mirror through the image-forming optical system has the intensity several to dozens times the typical intensity because the sunlight has been converged by the concave mirror in the image-forming optical system. Depending on the converging property of the concave mirror, the intensity of the sunlight might be five times or almost fifteen times the typical intensity. In view of this, the twice-reflected light ray of the sunlight is eliminated to prevent the adverse effect on the imaging of an enlarged virtual image.
To obtain appropriate conditions for eliminating the twice-reflected light rays, the radius of curvature and array pitch of the microlenses may be increased. This might reduce the pixel density and restrict the divergence angle of the diverging light beam, which reduces the effective viewing area (eye range), thus degrading image quality and usability.
Accordingly, by setting the conditions for preventing the sunlight, which has been even reflected twice by the micro-optical structure, from returning to the observation system, the occurrence of the postcard effect on the enlarged virtual image due to the light reflected by the microlens array can be substantially eliminated. In addition, even if the sunlight is reflected twice by the micro-optical structure, an arrangement of the optical components may be designed such that the twice-reflected light rays are blocked or the twice-reflected light rays are prevented from being reflected by the concave mirror within such a range that does not adversely affect the optical path of the projection light for imaging an enlarged virtual image. Such an improved layout may reduce or eliminate the occurrence of stray light.

In the above-described embodiment, the cases where the deflected light beam scans the diffusion screen two-dimensionally so as to write a two-dimensional image in the diffusion screen are described.
The two-dimensional scanning of the diffusion screen can also be performed by two-dimensionally moving the light beam in parallel, instead of deflecting light beam.
In the case of such a parallel movement, the scanning angle is 0. Accordingly, the maximum scanning angle θw is also 0.
Thus, the angle θm and the half-value divergence angle θs satisfy the conditional formula: θm> θs + 90° so as to achieve the advantageous effects of the embodiment of the present disclosure.
In the HUD described above, a two-dimensional image to be formed as an enlarged virtual image is formed by the light beam scanning the diffusion screen two-dimensionally to write a two-dimensional image thereon.
In the embodiments of the present disclosure, the image projection apparatus and the HUD are available without two-dimensionally scanning the diffusion screen with a light beam.
In other words, the image projection apparatus and HUD according to the embodiments of the present disclosure are applicable in a two-dimensional display apparatus incorporating the microlens array in the display, such as thin film transistor (TFT) display apparatus and a liquid display apparatus.
FIG. 13 is an example in which the diffusion screen according to an embodiment of the present disclosure is applied to a liquid crystal display apparatus.
In FIG. 13, the liquid crystal display apparatus includes a liquid crystal panel 50A and a diffusion screen 60. The diffusion screen 60 is formed in close contact with the upper surface of the liquid crystal panel 50A. In the liquid crystal panel 50A, light transmitting pixels and light blocking pixels are arranged in an array to form a two-dimensional image to be formed as an enlarged virtual image. When emitted to the liquid crystal panel 50A from the lower side, illumination light L0 passes through the two-dimensional array of the light emitting pixels, and enters the diffusion screen 60 as light for forming a two-dimensional image, which is referred to as image light.
The diffusion screen 60 transmits the image light incident on the diffusion screen 60 two-dimensionally therethrough, and converts the image light to diverging light with a divergence angle of 2θs. That is, a micro-optical structure is a two-dimensional array of microlenses, i.e., a microlens array formed at one side of the diffusion screen 60 so as to convert the image light two-dimensionally incident on the diffusion screen 60 into diverging light DLB0 with the divergence angle of 2θs.

The diverging light DLB0 with the divergence angle of 2θs serves as a flux of light rays for forming an enlarged virtual image. The diverging light DLB0 travel through the image-forming optical system including, for example, a concave mirror, which forms an enlarged virtual image. Thus, the formed enlarged virtual image is observed by the observer.
In the two-dimensional array of the micro-optical structure, such as a microlens array formed in the diffusion screen 60, when θm denotes an angle formed by tangent planes defined at reflection positions at which an external light ray SL other than the image light is reflected twice (the first reflection occurs at one reflection position, and the second reflection occurs at the other reflection position) by a microlens of the micro-optical structure so as to travel as a twice-reflected light ray LP to the side to which the diverging light DLB0 is emitted, and θs denotes a half-value divergence angle of the diverging light DLB0, the conditional formula below is satisfied:
θm > θs + 90°.

In this case as well, the exit angle θr of the light ray LP reflected twice by the micro-optical structure is greater than the divergence angle θs, thus preventing the adverse effect on the imaging of the enlarged virtual image.

Therefore, when θmax denotes a maximum angle of a diverging light ray diverged by the diffusion screen under the diffusion action of the surface formed by the two-dimensional array of the micro-optical structure, relative to a normal to a surface through which the diverging light ray exits the micro-optical structure; and θr denotes an exit angle of the twice-reflected light ray that has been reflected twice by the micro-optical structure and exits the micro-optical structure, the following conditional formula is satisfied so as to eliminate or reduce the reduction in the contrast of an enlarged virtual image due to the twice-reflected light rays:
θr ≧ θmax.

The maximum angle θmax corresponds to the above-mentioned “θs + θw + 90°” when a light beam is two-dimensionally deflected to write a two-dimensional image, and corresponds to the above-described “θs + 90°” when no scanning angle is produced.

Hereinafter, two examples of processing of the twice-reflected light ray LP are described with reference to FIGs. 14A and 14B. In FIGs. 14A and 14B, the light ray DLB0 is a light ray that has been caused to diverge by the diffusion screen 30, and has a divergence angle θs. A light ray LP is a twice-reflected light ray that has been reflected twice by the micro-optical structure of the diffusion screen 30, and travels to the side to which the light ray DLB0 is emitted. Further, optical components constituting the image-forming optical system includes a concave mirror 40. Hereinafter, the concave mirror 40 is also referred to as an image-forming optical system.
In FIG. 14A, the two-reflected light ray LP travels to the outside of the effective area of the image-forming optical system (the concave mirror 40), which does not have an influence on the imaging of an enlarged virtual image. However, in order to prevent the twice-reflected light ray LP from adversely acting as stray light, it is preferable that the inner surface of the housing 1A, the effective reflecting planes of the mirrors ML1 and ML2 are absorptive.

In FIG. 14B, a light shield 41 is provided outside the effective area of the image-forming optical system (the concave mirror 40) so as to block the twice-reflected light ray LP from entering the effective area.

With the above-described configuration in which the image-forming optical system captures light (diverging light beam/rays) with an angle less than the exit angle θr of the twice-reflected light ray LP, the twice-reflected light ray of the external light that has reached the diffusion screen can be prevented from traveling toward the observer. Thus, the reduction in the contrast of the enlarged virtual image can be effectively eliminated or reduced.

The present disclosure is not limited to the details of the example embodiments described above, and various modifications and improvements are possible.
The advantageous effects described in the embodiments of the present disclosure are preferred effects provided by disclosure, and the preferred effects are just recited; therefore, advantageous effects of the present disclosure are not limited to the effects described in the embodiments.

Numerous additional modifications and variations are possible in light of the above teachings. It is therefore to be understood that, within the scope of the above teachings, the present disclosure may be practiced otherwise than as specifically described herein. With some embodiments having thus been described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the scope of the present disclosure and appended claims, and all such modifications are intended to be included within the scope of the present disclosure and appended claims.

This patent application is based on and claims priority pursuant to 35 U.S.C. §119(a) to Japanese Patent Application No. 2018-154649, filed on August 21 in the Japan Patent Office, the entire disclosure of which is hereby incorporated by reference herein.

1 HUD
Im enlarged virtual image
50 windshield
Ob observer
A observer's eye
15 light deflector
30 diffuse screen
40 concave mirror
SL sunlight ray
LP twice-reflected light ray
θr exit angle of twice-reflected light ray

Claims

1. A diffusion screen (30) comprising:
a micro-optical structure (300) of a two-dimensional array, disposed on one side of the diffusion screen (30) to be two-dimensionally scanned with a light beam, wherein
the micro-optical structure (300) is configured to diverge the scanning light beam at a divergence angle of 2θs degrees so as to emit a diverging light beam, and reflect, as a twice-reflected light ray, a light ray other than the scanning light beam to a side to which the diverging light beam is emitted, and
wherein a conditional formula below is satisfied:
θm > θs + θw + 90 degrees
where
θm denotes an angle formed by tangent planes defined at two positions at which the light ray is reflected twice by the micro-optical structure (300),
θs denotes a half value of the divergence angle, and
θw is greater than or equal to 0, and denotes an effective maximum scanning angle of the scanning light beam.

A diffusion screen (30) comprising:
a micro-optical structure (300) of a two-dimensional array, disposed on one side of the diffusion screen (30), wherein
the micro-optical structure is configured to transmit image light for forming an image two-dimensionally emitted to the micro-optical structure, and diverge the image light at a divergence angle of 2θs degrees so as to emit a diverging light beam,
the micro-optical structure is further configured to reflect, as a twice-reflected light ray, a light ray other than the image light to a side to which the diverging light beam is emitted, and
wherein a conditional formula below is satisfied:
θm > θs + θw + 90 degrees
where
θm denotes an angle formed by tangent planes defined at two positions at which the light ray is reflected twice by the micro-optical structure (300), and
θs denotes a half value of the divergence angle.

The diffusion screen (30) according to claim 1 or 2,
wherein the micro-optical structure (300) is a microlens array in which microlenses are two-dimensionally disposed in array, and each of the microlenses has a concave surface, and
wherein external light rays are reflected twice by the concave surfaces of the microlenses so as to exit the micro-optical structure (300).

The diffusion screen (30) according to claim 1,
wherein the micro-optical structure (300) has concave surfaces on the one side to be two-dimensionally scanned with the light beam, and
wherein the scanning light beam is reflected by the concave surfaces of the micro-optical structure (300) at the divergence angle of 2θs degrees, and proceeds as a diverging light beam.

The diffusion screen (30) according to claim 4,
wherein the micro-optical structure (300) is a two-dimensional array of micro-concave reflecting surfaces, and
wherein external light rays are reflected twice by the micro-concave reflecting surfaces so as to exit the micro-optical structure (300).

The diffusion screen (30) according to any one of claims 1 to 5,
wherein a conditional formula below is satisfied:
θr ≧ θmax
where
θmax denotes a maximum angle of the diverging light beam diverged by the micro-optical structure (300) under a diffusion action of a surface of the two-dimensional array of the micro-optical structure (300), relative to a normal to an exit surface through which the diverging light beam exits the micro-optical structure (300), and
θr denotes an exit angle at which the twice-reflected light ray exits the micro-optical structure (300).

An image projection apparatus (1) comprising:
the diffusion screen (30) according to any one of claims 1 to 6; and
an image-forming optical system (ML1, ML2, and 40) configured to form an enlarged virtual image of a two-dimensional image on the diffusion screen (30),
wherein the divergence angle of 2θs of the diffusion screen (30) is set according to a boundary of an effective viewing area for an observer to identify the enlarged virtual image.

An image projection apparatus (1) comprising:
the diffusion screen (30) according to any one of claims 1, 4, and 5; and
an image-forming optical system (ML1, ML2, and 40) configured to form an enlarged virtual image of a two-dimensional image formed by two-dimensionally scanning the diffusion screen (30) with a light beam,
wherein the divergence angle of 2θs of the diffusion screen (30) is set according to a boundary of an effective viewing area for an observer to identify the enlarged virtual image.

An image projection apparatus comprising:
the diffusion screen (30) according to claim 2; and
an image-forming optical system (ML1, ML2, and 40) configured to form an enlarged virtual image of a two-dimensional image formed with the two-dimensionally emitted image light,
wherein the divergence angle of 2θs of the diffusion screen (30) is set according to a boundary of an effective viewing area for an observer to identify the enlarged virtual image.

The image projection apparatus according to any of claims 7 to 9, wherein
the image projection apparatus is a head-up display.

A mobile object (mo) comprising the HUD (1) according to claim 10.